Cycling the Wagons for Biliverdin Reductase

This letter was prompted by our reading of the article by Maghzal et al. (1). Bilirubin (BR) is a major physiologic antioxidant cytoprotectant. Tissue levels are only in nanomolar concentrations, because high levels are neurotoxic, yet BR protects against micromolar levels of oxidants because of a cycle wherein biliverdin (BV) generated by BR antioxidant action is efficiently reconverted by biliverdin reductase (BVR) to BR (2). Additional evidence for the cycle includes the extraordinary antioxidant potency of BR and augmented levels of reactive oxygen species (ROS), lipid oxidation, and susceptibility to cell death in BVR-depleted tissues (2, 3). Maines and colleagues (4) have confirmed that BVR depletion accentuates cytotoxicity elicited by arsenate stress. Stocker and associates (1) confirm some but not all of these observations. They fail to observe increased oxidant-induced cell death in BVR-depleted cells. However, they employ extremely high H₂O₂ concentrations that would be expected to overwhelm a physiologic antioxidant; they examine cells at 4–8 h post-oxidant, whereas the phase of cell death protected by BVR occurs at 18–24 h (2, 3). They fail to influence cellular ROS by overexpression of BVR (see Fig. 5 in Ref. 1) but do not include BV, the BVR substrate, so meaningful levels of BR may not be generated. Moreover, heme oxygenase is rate-limiting in BR formation so that overexpressing BVR without additional heme oxygenase should not influence BR levels. They maintain that oxidants only modestly convert BR to BV, but they use unfavorable conditions, including chloroform and dimethyl sulfoxide, and still some of their oxidants afford a 25% conversion, similar to what we observed (2). In summary, technical aspects of the Stocker study can account for discrepancies between their and our findings but do not meaningfully challenge the fundamentals. The BVR cycle has pathophysiologic as well as physiologic importance, and in numerous clinical studies bilirubin protects against cardiovascular and cerebrovascular disease (5).